U.S. patent application number 16/132965 was filed with the patent office on 2020-03-19 for method for determining at least one beam propagation parameter of a laser beam.
The applicant listed for this patent is TRUMPF Lasersystems for Semiconductor Manufacturing GmbH. Invention is credited to Viktor Granson, Jonathan Mueller, Dirk Sodtke.
Application Number | 20200088570 16/132965 |
Document ID | / |
Family ID | 69773999 |
Filed Date | 2020-03-19 |
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United States Patent
Application |
20200088570 |
Kind Code |
A1 |
Granson; Viktor ; et
al. |
March 19, 2020 |
METHOD FOR DETERMINING AT LEAST ONE BEAM PROPAGATION PARAMETER OF A
LASER BEAM
Abstract
The invention relates to a method for determining at least one
beam propagation parameter (M.sup.2, w.sub.0, .theta., Z.sub.0) of
a laser beam, comprising: directing the laser beam through a lens
arrangement towards a spatially resolving detector, imaging the
laser beam at a plurality of different focus positions (F1, . . . )
relative to the spatially resolving detector by adjusting a focal
length (f.sub.1, . . . ) of the lens arrangement, and determining
the at least one beam propagation parameter (M.sup.2, w.sub.0,
.theta., Z.sub.0) by evaluating an intensity distribution (l(x,y))
of the laser beam on the spatially resolving detector at the
plurality of different focus positions (F1, . . . ). The method
comprises adjusting the focal length (f.sub.1, . . . ) of the lens
arrangement by arranging lens elements (A1, . . . ; B1, . . . )
having different focal lengths (f.sub.A1, . . . ; f.sub.B1, . . . )
in a beam path of the laser beam.
Inventors: |
Granson; Viktor; (San Diego,
CA) ; Mueller; Jonathan; (Stuttgart, DE) ;
Sodtke; Dirk; (Tuebingen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TRUMPF Lasersystems for Semiconductor Manufacturing GmbH |
Ditzingen |
|
DE |
|
|
Family ID: |
69773999 |
Appl. No.: |
16/132965 |
Filed: |
September 17, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01J 1/0411 20130101;
B23K 26/402 20130101; G01M 11/061 20130101; B23K 26/705 20151001;
G01J 1/4228 20130101; G01J 1/0444 20130101; G01J 1/4257 20130101;
B23K 26/064 20151001; G02B 7/16 20130101 |
International
Class: |
G01J 1/42 20060101
G01J001/42; G01M 11/06 20060101 G01M011/06; B23K 26/064 20060101
B23K026/064; B23K 26/402 20060101 B23K026/402; G02B 7/16 20060101
G02B007/16 |
Claims
1. A method for determining at least one beam propagation parameter
of a laser beam, comprising: directing the laser beam through a
lens arrangement towards a spatially resolving detector, imaging
the laser beam at a plurality of different focus positions relative
to the spatially resolving detector adjusting a focal length of the
lens arrangement, and determining the at least one beam propagation
parameter by evaluating an intensity distribution of the laser beam
on the spatially resolving detector at the plurality of different
focus positions, characterized in that adjusting the focal length
of the lens arrangement comprises arranging lens elements having
different focal lengths in a beam path of the laser beam.
2. The method according to claim 1, wherein the lens arrangement
comprises at least one carrier, the carrier comprising a plurality
of accommodation spaces, each for accommodating at least one lens
element and/or at least one attenuation element, wherein arranging
the lens elements having different focal lengths in the beam path
of the laser beam comprises moving at least one carrier having a
plurality of accommodation spaces accommodating at least one lens
element relative to the beam path.
3. The method according to claim 2, wherein the lens arrangement
comprises at least two carriers arranged one behind the other along
the beam path of the laser beam, and wherein arranging the lens
elements having different focal lengths in the beam path of the
laser beam comprises moving the at least two carriers independently
from one another.
4. The method according to claim 2, wherein the at least one
carrier is designed as a rotatable magazine, and wherein arranging
the lens elements having different focal lengths in the beam path
of the laser beam comprises rotating the rotatable magazine.
5. The method according to claim 4, wherein the lens arrangement
comprises at least two carriers designed as rotatable magazines,
and wherein arranging the lens elements having different focal
lengths in the beam path of the laser beam comprises rotating the
at least two magazines independently from one another.
6. The method according to claim 4, wherein at least one rotatable
magazine comprises at least four accommodation spaces, and wherein
all accommodation spaces of the rotatable magazine are arranged
subsequently in the beam path of the laser beam for adjusting the
focal length of the lens arrangement.
7. The method according to claim 1, wherein the lens arrangement
comprises a housing for sealing an interior space in a light-tight
manner, and wherein directing the laser beam along the beam path
through the lens arrangement comprises directing the laser beam
through a first opening into the interior space of the housing and
through a second opening out of the interior space of the
housing.
8. The method according to claim 1, wherein the beam propagation
parameter of the laser beam is selected from the group consisting
of: beam propagation ratio, beam waist, far-field divergence angle,
and axial beam waist position.
Description
[0001] The invention relates to a method for determining at least
one beam propagation parameter of a laser beam, comprising:
directing the laser beam through a lens arrangement towards a
spatially resolving detector, imaging the laser beam at a plurality
of different focus positions relative to the spatially resolving
detector by adjusting a focal length of the lens arrangement, and
determining the at least one beam propagation parameter by
evaluating an intensity distribution of the laser beam on the
spatially resolving detector at the plurality of different focus
positions.
[0002] Determining beam propagation parameters of a (collimated)
laser beam. e.g. a beam propagation ratio (M.sup.2) may be
performed based on a method described in the ISO 11146 measurement
standard. According to the method, a lens of a known focal length
is held stationary and a sensor is moved through the waist of the
beam to take a series of measurements along a plurality of
different positions (e.g. ten or more positions) along the beam
propagation axis of the laser beam, determining a beam diameter for
each position to obtain a beam caustic of the laser beam and then
performing a curve fit to the measured data to calculate the
M.sup.2 parameter from that curve fit.
[0003] As the distance between the lens and the sensor has to be
varied, performing the ISO measurement method with a measurement
tool e.g. in a cleanroom is technologically involved. For instance,
the measurement tool needs to have a radiation-tight housing while
the distance between the lens and the sensor has to be varied,
requiring either an automated moving or complex mechanics to move
the detector inside the housing. For this reason, it may be
necessary to fold the beam path in order to keep the measurement
tool compact. Moreover, the fixation of the measurement tool to
e.g. a processing device or the like where the measurement tool is
used needs to be mechanically stable, etc.
[0004] DE 10 2015 014 387 B3 discloses a device for beam analysis
of light beams that comprises a variable optical element, an
objective lens, and a spatially resolving detector. The variable
optical element has an adjustable focal length and the objective
lens has a constant focal length. By changing the adjustable focal
length of the variable optical element and by subsequent focusing
of the light beam through the objective lens, a focus position of
the focused light beam relative to the spatially resolving detector
in an axial direction can be adjusted. In this way, a distance
between the objective lens and the spatially resolving detector may
be kept constant during the measurement. The variable optical
element may be a fluid lens, an adaptive lens or an adaptive
mirror. For instance, for CO.sub.2/mid IR lasers one may use
variable radius mirrors as adaptive mirrors, where the radius of
curvature is changed via water pressure. However, using such
mirrors greatly increases device complexity, cost and
maintenance.
OBJECT OF THE INVENTION
[0005] The present invention seeks to provide an improved method
for determining at least one beam propagation parameter of a laser
beam.
SUBJECT MATTER OF THE INVENTION
[0006] This object is achieved by a method of the type set forth at
the outset, wherein adjusting the focal length of the lens
arrangement comprises arranging lens elements having different
focal lengths in the beam path of the laser beam.
[0007] Rather than using a variable optical element for adjusting
the focal length as described in DE 10 2015 014 387 B3, the present
invention proposes adjusting the focal length of the lens
arrangement by arranging lens elements having different focal
lengths in the beam path of the laser beam. In this way, the lens
arrangement only allows adjusting a pre-defined number of
(different) focal lengths. Thus, the focal length of the lens
arrangement and consequently each focal position of the laser beam
relative to the spatially resolving detector is well-defined. A
further advantage of using a set of lenses with discrete focal
length is the robustness in industrial applications e.g. due to
abstinence of active control elements such as pressure regulators
of a variable focal length apparatus.
[0008] In contrast thereto, when using a variable optical element
such as a fluid lens, the focal length can be varied continuously,
so that it may be difficult to precisely define the focus position
when adjusting the focal length. Moreover, the variation range of
the focal length that can be adjusted by using a variable optical
element is typically rather small and technical realizations of
such adjustable optical elements may not exist for certain
wavelength or intensity regimes, e.g. in the IR wavelength range.
Finally, in contrast to the ISO measurement method, in the method
described above, the distance (optical path) between the lens
arrangement and the spatially resolving detector is kept
constant.
[0009] In one variant, the lens arrangement comprises at least one
carrier, the carrier comprising a plurality of accommodation
spaces, each for accommodating at least one lens element and/or at
least one attenuation element, wherein the step of arranging the
lens elements having different focal lengths in the beam path of
the laser beam comprises moving at least one earner having a
plurality of accommodation spaces accommodating at least one lens
element relative to the beam path of the laser beam. The movement
of the carrier relative to the beam path of the laser beam
typically involves moving a first accommodation space out of the
beam path and moving another accommodation space into the beam path
of the laser beam. In general, each accommodation space may be
configured to accommodate more than one lens element, the lens
elements typically being stacked in the direction of an optical
axis of the laser beam in this case. Preferably, each accommodation
space accommodates exactly one lens element.
[0010] In particular when the lens arrangement comprises more than
one carrier, one of the accommodation spaces may not accommodate a
lens element, so that the laser beam may pass through that
accommodation space without being focused or defocused by a lens
element. Typically, the lens elements of the lens arrangement are
focusing lenses. However, as the case may be, at least one of the
lens elements may be a diverging lens.
[0011] In addition to lens elements, the lens arrangement may
comprise power attenuation elements to homogenize the
signal/intensity levels of the laser beam being imaged at different
focus positions, leading to different spot sizes of the intensity
distribution on the spatially resolving detector. As the spot size
and hence the intensity level on the spatially resolving detector
depends on the focal length of a respective lens element, the
attenuation elements in general have different attenuation levels.
At least one attenuation element may be arranged together with a
respective lens element in one and the same attenuation space of
the carrier. In this case, the attenuation levels of the
attenuation elements are adapted to the focal length of the
corresponding lens elements so that the intensity level on the
detector is homogenized, thus reducing the dynamic range
requirement of the spatially resolving detector. The attenuation
elements may be transmissive optical elements e.g. in the form of
plane-parallel plates made of a material having a pre-defined
absorption level. Alternatively, the attenuation elements may be
devised as coatings on the lens elements made e.g. of an absorptive
coating material, as beam splitters, etc. In either case, the same
absorptive material may be chosen for all attenuation elements,
different attenuation levels being achieved by selecting
attenuation elements having different thicknesses.
[0012] In one variant, the lens arrangement comprises at least two
carriers arranged one behind the other along the beam path of the
laser beam, and wherein the step of arranging the lens elements
having different focal lengths in the beam path of the laser beam
comprises moving the at least two carriers independently from one
another. By using more than one carrier, the number of combinations
of lens elements arranged in the beam path and thus of different
focal lengths that may be adjusted by using the lens arrangement
may be considerably increased. For instance, when two carriers are
used and each carrier has four accommodation spaces, the total
number of optical configurations, resp., of different focal
positions is 4.times.4=16. Those skilled in the art will appreciate
that the overall (effective) focal length of the lens arrangement
depends on the focal lengths of the individual lens elements
arranged in the beam path of the laser beam and on the distance
between these lens elements, resp., between the carriers according
to the laws of geometrical optics. The focal lengths of the lens
elements of the lens arrangement may be selected in such a way that
a well-distributed spacing of imaging planes, resp., of overall
focal lengths is generated when the focal length of the lens
arrangement is adjusted.
[0013] In particular for the case that the lens arrangement
comprises two carriers, each having accommodation spaces
accommodating lens elements, a third carrier having accommodation
spaces that accommodate only attenuation elements may be provided.
In this way, the combinations of lens elements and attenuation
elements that are arranged in the beam path may be chosen
independently, thus reducing the number of attenuation elements
needed for the homogenization of the intensity level at the
spatially resolving detector.
[0014] In a preferred variant, the at least one carrier is designed
as a rotatable magazine and the step of arranging the lens elements
having different focal lengths in the beam path of the laser beam
comprises rotating the rotatable magazine. Using a carrier in the
form of a rotatable magazine, e.g. in the form of a disc-shaped
magazine (turret wheel) having a plurality of accommodation spaces
arranged at an equal distance from a center axis (revolution axis)
of the magazine allows to provide a compact lens arrangement. The
accommodation spaces are typically arranged at an equal spacing in
the circumferential direction. For instance, when the rotatable
magazine has four accommodation spaces, these are typically
arranged at an angle of 90.degree. relative to one another in the
circumferential direction. The lens arrangement may comprise an
actuator for rotating the disc-shaped magazine around the
revolution axis to arrange one of the accommodation spaces in the
beam path of the laser beam. Alternatively, the disc-shaped
magazine may be rotated manually from the exterior of the housing,
e.g. by providing a recess or the like in the housing so that a
cylindrical outer edge of the disc-shaped magazine is accessible
for an operator in order to rotate the magazine. In this way, the
need for a motorized wheel is eliminated, thus improving robustness
of the lens arrangement to failures.
[0015] An evaluation unit that is in signal communication with the
detector may activate the actuator at pre-defined times to change
the position of the accommodation spaces, thus allowing to relate
an intensity distribution detected on the spatially resolving
detector to a corresponding focus position.
[0016] One skilled in the art will appreciate that the carrier
comprising the plurality of accommodation spaces is not necessarily
designed as a disc-shaped magazine, but may be designed e.g. in the
form of a (linear) slide for moving different accommodation
spaces/lens elements into the beam path by translating the slide,
typically in a direction perpendicular to a propagation axis of the
laser beam. In this respect, reference is made to DE 196 55 127 C2,
describing a connecting head for processing a workpiece with a
laser beam, the connecting head comprising an automated changing
mechanism for changing focusing optics, the changing mechanism
having at least one movable carrier with a plurality of
accommodation spaces.
[0017] US 2017/0062247 A1 describes an optical station for
exchanging optical elements having a rotatable magazine with a
plurality of accommodation spaces for accommodating a plurality of
holders for holding respective optical elements. One skilled in the
art will appreciate that the accommodation spaces of the carrier(s)
of the lens arrangement described herein may likewise comprise
holders for holding the optical elements, and that the lens
elements may be removed from the accommodation spaces together with
the holders e.g. in the way described in US 2017/0062247 A1.
[0018] In one development, the lens arrangement comprises at least
two carriers designed as rotatable magazines, and the step of
arranging the lens elements having the different focal lengths in
the beam path of the laser beam comprises rotating the at least two
magazines independently from one another. As indicated above, the
rotatable magazines are arranged along the propagation axis of the
laser beam one after the other. By independently rotating the two
disc-shaped magazines, the number of focal lengths that may be
adjusted can be increased considerably without having to
significantly increase the installation space of the lens
arrangement.
[0019] In another variant, at least one rotatable magazine
comprises at least four accommodation spaces and all accommodation
spaces of the rotatable magazine are subsequently arranged in the
beam path of the laser beam for adjusting the focal length of the
lens arrangement. As indicated above, it is advantageous to arrange
all available lens elements/accommodation spaces in the beam path
of the laser beam in order to increase the number of different
focal positions that can be adjusted with the lens arrangement.
[0020] In another variant, the lens arrangement comprises a housing
for sealing an interior space in a light-tight manner, and wherein
the step of directing the laser beam through the lens arrangement
comprises directing the laser beam through a first opening into the
interior space of the housing and through a second opening out of
the interior space of the housing. Preferably, the first opening
and the second opening are aligned along a line of sight parallel
to the propagation axis of the laser beam. In this way, the laser
beam can pass through the lens arrangement without a lateral
displacement, reducing the installation space of the lens
arrangement. Optionally, windows in the form of plane-parallel
plates that are aligned perpendicular to the propagation axis of
the laser beam may be arranged in the openings. In any case, the
lens arrangement/tool having the light-tight housing should enclose
the laser beam entirely to be classified as a laser safety class 1
device, making the tool safe under all conditions of normal
use.
[0021] The lens arrangement comprising the light-tight housing can
be used as mobile measurement tool in a cleanroom or the like, e.g.
for determining at least one beam propagation parameter of a laser
beam that is used e.g. in a laser amplifier arrangement or in an
EUV radiation generating device comprising such a laser amplifier
arrangement. In a EUV radiation generating device, a laser beam is
guided from the laser amplifier arrangement via a beam guiding
device to a vacuum chamber. By irradiating a target material
arranged in the vacuum chamber, e.g. in the form of tin droplets, a
plasma can be generated, the plasma emitting EUV radiation. It will
be appreciated that the use of the (mobile) measurement tool is not
limited to determining beam propagation parameters of laser beams
of driver laser arrangements for EUV radiation generation, but also
in other installations, e.g. in processing machines for laser
processing of workpieces or the like.
[0022] In a further variant, the beam propagation parameter of the
laser beam is selected from the group consisting of: beam
propagation ratio, beam waist (i.e. minimum beam radius), axial
beam waist position (i.e. axial position along the beam bath with
minimum beam radius), far-field divergence angle, or other
representations of these or other beam propagation parameters or
quantities derived therefrom, e.g. combinations of the beam
parameters described herein or of other beam propagation
parameters. For instance, instead of the axial beam waist position
z.sub.0, the beam radius (or diameter) at another axial position
together with the information if the laser beam is diverging or
converging at that axial position constitutes a derived quantity
that is equivalent to the axial beam waist position, i.e. that
provides the same information about the beam propagation of the
laser beam. In a similar way, the beam propagation ratio and the
far-field divergence angle are equivalent parameters for a given
wavelength of the laser beam.
[0023] The beam propagation ratio M.sup.2--often referred to as
beam quality factor--represents the degree of variation of a beam
from an ideal Gaussian beam. The beam propagation ratio M.sup.2 is
defined as the ratio of the far-field divergence angle of the laser
beam with a given beam waist in relation to the far-field
divergence angle of an ideal Gaussian beam with the same beam
waist. However, as will be described below, it is not necessary to
determine the divergence angle close to the actual beam waist
directly in order to determine the beam propagation ratio
M.sup.2.
[0024] For determining beam propagation parameters) of the laser
beam, typically the intensity distribution of the laser beam on the
spatially resolving detector is evaluated to determine a beam
diameter of the laser beam for each of the optical configurations
of the lens arrangement, corresponding to different focus/imaging
positions. In this way, a "virtual" beam caustic is generated, i.e.
a series of images of the laser beam corresponding to different
known image planes with different known magnifications, from which
the beam diameter for the corresponding object planes can be
calculated by applying the laws of geometrical optics. The
"virtual" beam caustic formed by these calculated beam diameters at
the object planes may be evaluated in an analogous way as a beam
caustic measured in accordance with the ISO measurement standard,
e.g. by performing a curve fit to the measured data/beam diameters
to calculate the beam propagation ratio M.sup.2 and/or other
parameters such as the beam waist (minimum beam radius), the
far-field divergence angle of the laser beam, the axial beam waist
position, etc.
[0025] Further advantages of the invention emerge from the
description and the drawings. Likewise, the features mentioned
above and the features yet to be explained below may find use,
either respectively on their own or in any combination when a
plurality thereof are grouped together. The shown and described
embodiments should not be understood to be a comprehensive list
but, instead, should be seen to have an exemplary character for
explaining the invention.
[0026] In the FIGURES:
[0027] FIG. 1a,b show schematic illustrations of a lens arrangement
having an adjustable focus length for focusing a laser beam at a
first focus position and at a second focus position relative to a
spatially resolving detector.
[0028] In the following description of the drawings, identical
reference signs are used for identical or functionally identical
components.
[0029] FIG. 1a,b show a portable measurement tool in the form of a
lens arrangement 1. The lens arrangement 1 has a light-tight
housing 2 through which a collimated laser beam 3 is directed
towards a spatially resolving detector 4. e.g. a CCD-Camera, a CMOS
chip or another pixel-based light-sensitive detector. The laser
beam 3 has a beam axis 5 that is directed along a z direction of a
xyz coordinate system. The lens arrangement 1 is configured to
focus the laser beam 3 at different focus positions F1, F2, . . .
along the beam axis 5 and thus at different distances D1, D2, . . .
relative to the spatially resolving detector 4. For this purpose,
the lens arrangement 1 is configured for adjusting its focal length
f.sub.1, f.sub.2, . . . in a way that will be described in greater
detail below.
[0030] As can be gathered from FIG. 1a,b, the lens arrangement 1 is
arranged at a constant distance D from the detector 4. The distance
D is chosen so that the lens arrangement 1 focuses the laser beam 3
at a first focus position F1 upstream of the detector 4 and at a
second focus position F2 downstream of the detector 4 along the
beam axis 5 of the laser beam 3. It will be appreciated that the
two focus positions F1, F2 are shown for illustrative purposes
only, and that the lens arrangement 1 is configured to focus the
laser beam 3 at a plurality (typically ten or more) different focus
positions F1, F2, . . . . At each focus position F1, F2 an
intensity distribution I(x,y) perpendicular to the beam axis 5 of
the laser beam 3 is detected at a light-sensitive surface of the
detector 4. A programmable evaluation unit 6 that is in signal
connection to the detector 4 evaluates the intensity distribution
I(x, y) for each focus position F1, F2, . . . and determines a beam
diameter of the intensity distribution I(x, y) that corresponds to
a beam diameter of the laser beam 3 at the respective focus
position F1, F2 Based on the (known) distances D1, D2, . . .
between the focus positions F1, F2, . . . and the light-sensitive
surface of the detector 4, a beam caustic of the laser beam 3,
resp., beam propagation parameters of the laser beam 3 can be
determined.
[0031] For instance, by performing a curve fit of the beam
diameters determined by the evaluation unit 6 e.g. to a hyperbolic
function, the M.sup.2 parameter (beam propagation ratio) and other
parameters such as the beam waist w.sub.0 (minimal beam radius), a
far-field divergence angle .theta. of the laser beam 3, an axial
position z.sub.0 of the beam waist, etc. can be determined in a
similar way as in the ISO measurement method. It will be understood
that a variety of other methods can be used to analyze or evaluate
the measured beam diameters/intensity distributions I(x, y) at the
different focus positions F1, F2 to determine beam propagation
parameters M.sup.2, w.sub.0, .theta., z.sub.0, . . . of the laser
beam 3.
[0032] For adjusting the focal length f.sub.1, f.sub.2, . . . of
the lens arrangement 1, lens elements A1 to A4, B1 to B4 having
different focal lengths f.sub.A1, . . . , f.sub.A4; f.sub.B1, . . .
, f.sub.B4 are arranged in a beam path 7 of the laser beam 3
passing through the lens arrangement 1. The laser beam 3 enters
through a first opening 8a into an interior space 9 of the housing
2 of the lens arrangement 1 and the exits from the interior space 9
of the housing 2 through a second opening 8b towards the detector
4. In the example shown in FIG. 1a,b, the beam path 7 of the laser
beam is confined to the area between the first and second openings
8a, 8b of the housing 2.
[0033] The lens arrangement 1 has a first rotatable disc-shaped
magazine 10 and a second rotatable disc-shaped magazine 11,
arranged one after the other along the propagation direction z of
the laser beam 3. Each disc-shaped magazine 10, 11 may be rotated
around a common revolution axis 12 arranged at the center of the
respective magazine 10, 11 via an actuator e.g. in the form of a
rotation motor or manually by an operator. In the latter case, a
handling equipment may be provided that allows an operator to
rotate a respective magazine 10, 11 from the outside of the housing
2. For instance, a recess/opening may be provided in the housing 2,
allowing to access the circumferential edge of a respective
disc-shaped magazine 10, 11 for the rotation. In this case, care
must be taken to ensure that the housing 2 is still light-tight. In
either case, the disc-shaped magazines 10, 11 can be rotated
independently from each other either manually or by using two
independent actuators.
[0034] The first disc-shaped magazine 10 in the propagation
direction z of the laser beam 3 has four accommodation spaces
13a-d, two of which are shown in FIG. 1a and two of which are shown
in FIG. 1b. Each accommodation space 13a-d of the first disc-shaped
magazine 10 holds one lens element A1 to A4. In addition, each
accommodation space 13a-d of the first disc-shaped magazine 10
holds one attenuation element G1 to G4 for attenuating/reducing the
power of the laser beam 3. The attenuation elements G1 to G4 are
plane-parallel plates that have a different attenuation level
depending on the focal length f.sub.A1 to f.sub.A4 of the
corresponding lens element A1 to A4.
[0035] In an analogous way, the second disc-shaped magazine 11 in
the propagation direction z of the laser beam 3 has four
accommodation spaces 14a-d, each holding one lens element B1 to B4.
In addition, each of the accommodation spaces 14a-d also holds one
attenuation element H1 to H4. Each of the attenuation elements H1
to H4 generates a different level of attenuation of the power of
the laser beam 3, the attenuation level being dependent on the
focal length f.sub.B1 to f.sub.B4 of the corresponding lens element
B1 to B4. The accommodation spaces 13a-d, 14a-d are aligned at
angles of 90.degree. relative to one another in the xy plane
perpendicular to the propagation direction z of the laser beam
3.
[0036] In FIG. 1a, a first accommodation space 13a and thus a first
lens element A1 of the first magazine 10 is arranged in the beam
path 7 of the laser beam 3. In a similar manner, a first
accommodation space 14a and thus a first lens element B1 of the
second magazine 11 is arranged in the beam path 7 of the laser beam
3. The overall focal length f.sub.1 of the lens arrangement 1 of
FIG. 1a depends on the focal length f.sub.A1 of the first lens
element A1 of the first magazine 10, on the focal length f.sub.B1
of the first lens element B1 of the second magazine 11 and on the
(constant) distance between the two lens elements A1, B1 along the
propagation axis 5 of the laser beam 3 in accordance with the laws
of geometrical optics (not reproduced here).
[0037] For adjusting/changing the focal length of the lens
arrangement 1, the evaluation unit 6 or an operator acts on both
rotatable magazines 10, 11 to rotate these by an angle of
90.degree., as indicated by the arrows in FIG. 1a. After the
rotation, the second accommodation space 13b and thus the second
lens element A2 of the first magazine 10 is arranged in the beam
path 7 of the laser beam 3. Similarly, the second accommodation
space 13b and thus the second lens element B2 of the second
magazine 11 is arranged in the beam path 7 of the laser beam 3
after the rotation. Consequently, the lens arrangement 1 shown in
FIG. 1b has an overall focal length f.sub.2 that depends on the
focal length f.sub.A2 of the second lens element A2 of the first
magazine 10 and on the focal length f.sub.B2 of the second lens
element B2 of the second magazine 11. It will be appreciated that
by rotating the first and/or the second magazine 10, 11, sixteen
different focal lengths f.sub.1, f.sub.2, . . . may be adjusted
with the lens arrangement 1 of FIG. 1a,b. Such a number of
different focal lengths f.sub.1, f.sub.2, . . . is typically
sufficient to determine the beam propagation parameters M.sup.2, .
. . of the laser beam 3 with sufficient accuracy.
[0038] The attenuation level of a respective attenuation element G1
to G4, H1 to H4 is chosen so that the intensity levels of the
intensity profile I(x, y) on the spatially resolving detector 4 are
homogenized. Thus, when the focal length f.sub.A1 to f.sub.A4,
f.sub.B1 to f.sub.B4 of a respective lens element A1 to A4. B1 to
B4 generates an intensity profile I(x,y) on the spatially resolving
detector 4 that has a small spot size and thus a large intensity
level, the attenuation of the corresponding attenuation element G1
to G4, H1 to H4 will be high compared to the case when the
intensity profile I(x, y) has a larger spot size.
[0039] As an alternative to the embodiment shown in FIG. 1a,b,
instead of attenuation elements G1 to G4, H1 to H4 that are
arranged in one and the same accommodation space 13a-d, 14a-d
together with a corresponding lens element A1 to A4, B1 to B4, a
third rotatable magazine may be arranged in the housing 2 of the
lens arrangement 1. The third rotatable magazine may also comprise
four accommodation spaces, each holding an attenuation element, but
no lens element. The third rotatable magazine may be rotated
independently from the first and second rotatable magazine 10, 11,
so that different combinations of lens elements A1 to A4, B1 to B4
and attenuation elements can be arranged together in the beam path
7 of the laser beam.
[0040] In summary, the lens arrangement 1 described above
constitutes a compact portable measurement tool that may be used
e.g. In a cleanroom to determine beam propagation parameters of a
laser beam 3 with high accuracy. Moreover, as the case may be, the
need to provide mechanical actuators in the lens arrangement 1 for
acting on the rotatable magazines 10, 11 may be dispensed with.
* * * * *